In the related art, for example, in order to improve the power generation efficiency of coal-fired plants, integrated gasification combined cycle (IGCC) plants have been developed and introduced into practical use. An IGCC plant generates synthesis gas by gasifying carbon-containing fuel such as coal by means of a gasifier. The IGCC plant includes a gas turbine that is driven by using combustible gas obtained by refining synthesis gas, which is obtained by gasifying coal by the gasifire, by gas clean-up equipment as a fuel, and also includes a steam turbine that is driven by steam obtained by heat recovery from the gas turbine exhaust gas.
In the IGCC plant, during normal operation or when the load varies, the air ratio is controlled to become a predetermined set value for each load. The air ratio is the ratio of the amount of air that is supplied to the gasifier relative to the theoretical amount of air for combustion of coal. The flow rate of air or oxygen that is supplied to the gasifier as an oxidizer is controlled based on a gasifier input demand (GID), which is a parameter dictating heat input to the gasifier, as shown in FIG. 9.
The flow rate of coal that is supplied to the gasifier is controlled by using a set value based on the GID or a preceding signal on the occasion of a load variation.
The pressure of the gasifier (gasifier pressure) is controlled by using a set value based on a megawatt demand (MWD) of the IGCC plant, as shown in FIG. 10. In the case where a deviation from the set value occurs in the gasifier pressure, the gasifier pressure is controlled by increasing or decreasing the GID so as to match a set pressure value based on the MWD. As the load of the IGCC plant becomes larger, the gasifier pressure is set to be higher, and the pressure of the fuel that is supplied to the gas turbine increases.
In particular, in the case where the type of control described above is exercised in an IGCC plant in which a portion of air at the outlet of an air compressor of the gas turbine is extracted and is used as an oxidizer for the gasifier, there are cases where an operating-state quantity of the gasifier varies since the coal quality is not uniform while the MWD is constant, which results in a temporary reduction in the heating value of the synthesis gas.
Also, in the case where the type of control described above is exercised in an IGCC plant in which char that has been collected by dust removal equipment provided downstream of the gasifier is returned to and recycled in a char burner of the gasifier, there are cases where an operating-state quantity of the gasifier varies due to clogging of a char recycle system, etc. while the MWD is constant, which results in a temporary reduction in the heating value of the synthesis gas.
In such cases, the following event (hereinafter referred to as a “fuel-consumption increase event”) occurs in the IGCC plant.
Specifically, a reduction in the heating value of the synthesis gas causes an increase in fuel consumption to compensate for fuel heat input to the gas turbine. The increase in the fuel consumption of the gas turbine transiently causes a reduction in the gas pressure on the upstream side, causing a reduction in the gasifier pressure. In the case where a deviation occurs such that a measured value is less than the set pressure value of the gasifier, the GID is increased to increase a amount of introduction of coal and oxidizer.
In the IGCC plant in which a portion of air at the outlet of the air compressor of the gas turbine is extracted and is used as an oxidizer for the gasifier, an increase in the oxidizer flow rate causes an increase in the flow rate of air extracted from the gas turbine. The increase in the flow rate of the extracted air results in a reduction in the output of the gas turbine since air that serves as an operating medium of the gas turbine is transiently allocated by extracted air. The reduction in the output of the gas turbine causes the MWD to drop below the set value, and the fuel consumption of the gas turbine increases so as to compensate for the reduction.
As described above about the fuel-consumption increase event, mainly due to variations in the heating value of the synthesis gas, hunting occurs in the power generation output of the IGCC plant. Then, finally, as heat input to the gasifier increases, the amount of steam generated by a syn gas cooler (SGC) connected downstream of the gasifier increases, so that the output of the steam turbine increases. As a result, a command for decreasing the output of the gas turbine is issued, whereby the IGCC plant is stabilized.
FIG. 11 is a diagram showing temporal changes in various state quantities in the above fuel-consumption increase event. As shown in FIG. 11, in the case where a transient operating state occurs such that the heating value of the synthesis gas decreases, the gas turbine needs a greater amount of fuel gas in order to satisfy the MWD, so that the gasifier pressure begins to decrease (Refer to GT in in FIG. 11). The reduction in the gasifier pressure causes an increase in the flow rate of the coal that is supplied to the gasifier based on the GID. Then, the flow rate of the oxidizer (the flow rate of the air) that is supplied into the gasifier increases, together with an increase in the coal flow rate, so as to maintain an operating state in which coal ash is discharged in the form of molten slag and to maintain the air ratio, which represents the operating state most straightforwardly, to a predetermined set value for the purpose of stable operation of the gasifier. That is, the flow rate of air that is extracted from the air compressor to the gasifier increases.
Because of this change in the state quantity, there are cases where an overshoot occurs in the oxidizer flow rate in the process of stabilizing control. Thus, it is necessary to take into consideration an increase in the capacity corresponding to the overshoot (overshoot tolerance) regarding the capacity of oxidizer supply equipment (e.g., an air booster provided between the air compressor of the gas turbine and the gasifier). Furthermore, in the case where the above fuel-consumption increase event occurs, there are cases where it takes time to stabilize control of the IGCC plant as a whole due to the occurrence of an overshoot.
The increase in the capacity of the supply equipment due to the overshoot tolerance of the oxidizer results in increased device costs. Furthermore, as the disparity between the operating point during normal operation and the equipment design point considering the overshoot tolerance becomes greater, the motive power of the compressor during normal operation deviates from an optimal value (minimum value), so that an extra motive power is required.
Furthermore, when the load of the IGCC plant increases, as the fuel consumption of the gas turbine increases transiently, the pressure of the fuel gas system from the gasifier to the inlet of the gas turbine decreases. Furthermore, as the load of the IGCC plant increases, the set value of the gasifier pressure also increases, so that the pressure deviation of the fuel gas system tends to increase. Therefore, in order to reduce the deviation of the pressure of the fuel gas system from the set gasifier pressure, coal and oxidizer are supplied to the gasifier in advance according to a preceding signal, as shown in FIG. 12.
Also in this case, the above fuel-consumption increase event occurs due to the increase in the oxidizer flow rate, so that it takes time to stabilize control of the IGCC plant as a whole. Furthermore, it is also necessary to consider an overshoot tolerance, based on the preceding signal, about the capacity of oxidizer supplying equipment.
Here, Patent Literature 1 discloses a coal gasifier that maintains the heating value of the synthesis gas substantially constant by maintaining the amount of air supply serving as an oxidizer substantially constant while adjusting the amount of coal supply in the case where the heating value of the synthesis gas varies.